Various applications, including in laboratory, industrial, and audio applications, for example, share the need to measure the difference between two relatively weak individual voltages superimposed with a potentially very large common voltage. Instrumentation amplifiers fill this need by rejecting the superimposed common voltage, i.e. common mode voltage, on two individual input voltages, and generating an output voltage that is directly proportional to the difference between the two individual input voltages. Thus, the output voltage, VOUT, of an instrumentation amplifier is characterized by the operation of a common mode gain, ACM, and a differential gain, ADM, on the two input voltages, V1 and V2, according to equation 1.VOUT=ADM(V1−V2)+ACM(V1+V2)/2  (Eq. 1)
An ideal instrumentation amplifier has a constant differential gain and a common mode gain of zero. In practice, the common mode gain may not be zero but instead may be much less than the differential gain. An important measurement for instrumentation amplifiers is the common mode rejection ratio (CMRR), which is a ratio of the common mode gain to the differential gain expressed in decibels (dB). Typical instrumentation amplifiers may have CMRR in the range of 30 to 60 dB. The higher the CMRR, the closer an instrumentation amplifier is to ideal.
Typical instrumentation amplifiers are implemented using operational amplifiers and four or more resistors. The magnitude of resistance presented by particular resistors in the instrumentation amplifier dictates the gain of the amplifier. However, controlling the gain of these amplifiers through resistor selection is typically contingent upon matching a pair of resistors that have as close to identical resistance as is feasible. The difficulty and cost of fabricating an instrumentation amplifier may rise rapidly as greater precision is sought in matching the resistor pairs. For example, matching resistors above a certain level of precision typically requires an expensive process of using lasers for finely calibrated trimming of the resistors.
Resistors in instrumentation amplifiers may pose additional complications. For instance, the resistors may be implemented as polysilicon resistors that may have voltage coefficients that create significant distortion or non-linearity in the output voltage, even when resistor pairs are matched perfectly. Such voltage coefficients allow any common mode voltage to effectively modulate the output voltage by changing the differential gain.